12). In view of these considerations it is necessary to have at our disposal other forms of expression by which the direction of the current of response can still be designated. Keeping in touch with the old phraseology, we might then call a current ‘negative’ that flowed from the more excited to the less excited. Or, bearing in mind the fact that an uninjured contact acts as the zinc in a voltaic couple, we might call it ‘zincoid,’ and the injured contact ‘cuproid.’ Stimulation of the uninjured end, approximating it to the condition of the injured, might then be said to induce a cuproid change.
The electric change produced in a normal nerve by stimulation may therefore be expressed by saying that there has been a negative variation, or that there was a current of action from the more excited to the less excited, or that stimulation has produced a cuproid change.
The excitation, or molecular disturbance, produced by a stimulus has thus a concomitant electrical expression. As the excitatory state disappears with the return of the excitable tissue to its original condition, the current of action will gradually disappear.[3] The movement of the galvanometer needle during excitation of the tissue thus indicates a molecular upset by the stimulus; and the gradual creeping back of the galvanometer deflection exhibits a molecular recovery.
This transitory electrical variation constitutes the ‘response,’ and its intensity varies according to that of the stimulus.
Electric recorder.—We have thus a method of obtaining curves of response electrically. After all, it is not essentially very different from the mechanical method. In this case we use a magnetic lever (fig. 4, a), the needle of the galvanometer, which is deflected by the electromagnetic pull of the current, generated under the action of stimulus, just as the mechanical lever was deflected by the mechanical pull of the muscle contracting under stimulus.
The accompanying diagram (fig. 4, b) shows how, under the action of stimulus, the current of rest undergoes a transitory diminution, and how on the cessation of stimulus there is gradual recovery of the tissue, as exhibited in the return of the galvanometer needle to its original position.
Fig. 4.—Electric Recorder
(a) M muscle; A uninjured, B injured ends. E E′ non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces ‘negative variation’ of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery.
(b) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.
Two types of response—positive and negative.—It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation.
The following is a tabular statement of the two types of response:
I. Negative variation.—Action current from more excited to less excited—cuproid change in the excited—e.g. fresh muscle and nerve, stale retina.
II. Positive variation.—Action current from less excited to more excited—zincoid change in the excited—e.g. stale nerve, fresh retina.[4]
From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the sign plus or minus being a minor consideration.
Universal applicability of the electrical mode of response.—This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical.
The two response-curves seen in the accompanying diagram (fig. 5), and taken from the same muscle by the two methods simultaneously, clearly exhibit this. Thus we see that electrical response can not only take the place of the mechanical record, but has the further advantage of being applicable in cases where the latter cannot be used.
Electrical response: A measure of physiological activity.—These electrical changes are regarded as physiological, or characteristic of living tissue, for any conditions which enhance physiological activity also, pari passu, increase their intensity. Again, when the tissue is killed by poison, electrical response disappears, the tissue passing into an irresponsive condition. Anæsthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response.
Fig. 5.—Simultaneous Record of the Mechanical (M) and (E) Electrical Responses of the Muscle of Frog. (Waller.)
From these observed facts—that living tissue gives response while a tissue that has been killed does not—it is concluded that the phenomenon of response is peculiar to living organisms.[5] The response phenomena that we have been studying are therefore considered as due to some unknown, super-physical ‘vital’ force and are thus relegated to a region beyond physical inquiry.
It may, however, be that this limitation is not justified, and surely, at least until we have explored the whole range of physical action, it cannot be asserted definitely that a particular class of phenomena is by its very nature outside that category.
Electric response in plants.—But before we proceed to the inquiry as to whether these responses are or are not due to some physical property of matter, and are to be met with even in inorganic substances, it will perhaps be advisable to see whether they are not paralleled by phenomena in the transitional world of plants. We shall thus pass from a study of response in highly complex animal tissues to those given under simpler vital conditions.
Electric response has been found by Munck, Burdon-Sanderson, and others to occur in sensitive plants. But it would be interesting to know whether these responses were confined to plants which exhibit such remarkable mechanical movements, and whether they could not also be obtained from ordinary plants where visible movements are completely absent. In this connection, Kunkel observed electrical changes in association with the injury or flexion of stems of ordinary plants.[6] My own attempt, however, was directed, not towards the obtaining of a mere qualitative response, but rather to the determination of whether throughout the whole range of response phenomena a parallelism between animal and vegetable could be detected. That is to say, I desired to know, with regard to plants, what was the relation between intensity of stimulus
